Most personal computers display a brief message about entering System Setup soon after the power is turned on. System Setup is also called the “System BIOS” or sometimes the “CMOS Setup” – each is a name for the same thing.
For example, on Dell systems you will see “F2=Setup” in the upper right hand corner during the computer boot-up. This means to press the F2 key to enter Setup (the BIOS). Different computer manufactures have different methods to enter Setup, so please check your system documentation for specifics. How to access/enter Motherboard BIOS
System setup is where the date and time are stored and where startup preferences like NumLock or Passwords are defined. In addition, many hardware settings are defined in System Setup.
In particular, hard disk drives are often controlled here. At the simplest level, hard drives are numbered in Setup as Drive 0, Drive 1, etc or sometimes as Primary Master, Primary Slave. Which ever way these are named, they have a basic ON / OFF setting (sometimes AUTOMATIC or something similar). If your drive is not detected it may be because it is turned OFF in System Setup.
In most cases, the boot drive is Drive 0 or Primary Master. When you add a second drive to the system, it may be need to be enabled by turning it ON in System Setup.